Check the wattage of your power supply. For example if it was 400W then that would be 0.4kWh. A unit of electricity is one kWh and costs c. €0.14 including VAT (urban domestic rate - other tariffs have different charges). Thus it would cost c. 0.4 x €0.14 = €0.06 per hour to have the PC switched on. I'm assuming that the monitor would be off here. This is not exact but it's the general gist of how to calculate running costs for many devices. Why not enable and use hibernate mode to speed up "boot" times? Leaving it on unnecessarily is a waste.